LiFePO4 balancing / voltage limiting

So I just replaced my old AGM bank with a bank of 16 180ah CALB cells. I bottom balanced them; paralleled them all and brought them down to 2.70 volts. Let it rest for 2 days and they rebounded to 2.73. Disconnected the parallel connections for another day and they all stayed at 2.73.

Then I seriesed them and did the first charge while monitoring. They tracked very closely for the CC portion of charging. (Charger 20 amps, 56 volts, targeting 3.5 volts per cell.) When they switched to CV I measured again and got:

3.52 3.45

3.47 3.43

3.48 3.43

3.44 3.45

3.49 3.58

3.50 3.49

3.50 3.47

3.40 3.49

I was a little worried about cell 13 (3.58) but it was still well below 3.65V and I figured charge was almost complete. I checked 15 minutes later, when current had dropped to 2 amps, and saw:

3.59 3.41

3.45 3.39

3.50 3.39

3.41 3.43

3.52 3.75

3.63 3.40

3.55 3.44

3.36 3.40

So I stopped charge and discharged it until cell 13 was down in the safe range again (3.5 volts.) I let it sit overnight and now everything is back at 3.33 to 3.35 volts. Hopefully no harm was done to 13.

So how do I handle cell 13? Do I have to monitor voltages at all cells and discontinue charging when it hits 3.6V or so? Do I back off the 56 volt charge, go down to 54 volts or something? Do I terminate when current reaches C/20 or something? (hard to do with this charger)

Get a 6 volt brake (or tail) light and clip it on the high voltage cell, and monitor until it gets down to the average pack voltage.

I explicitly wanted to NOT do that because then they're not bottom balanced any more. Right now, in theory, during discharge when they hit 2.7 volts they should do it all at the same time - so that if I set LVD to 45 volts the cells should be safe against overdischarge. If I discharge that cell then it will hit 2.7 volts much earlier than the rest, and be at risk for overdischarge.

I'm a no-nothing on these, but that seems like a pretty wide range (3.36-3.75) for cells charged once in series from an apparently equal SOC. My understanding is voltage should be pretty much flat between ~10-90%SOC. That implies when the highest voltage cell gets to whatever 90%SOC voltage, stop charging. Discharge until the lowest gets to 10%SOC voltage. The difference is capacity.

The problem is that you cannot have an LFP battery balanced at both the top and bottom unless all the cells have exactly the same capacity which is unlikely. If you have it bottom balanced you have to use a BMS to terminate the charge when the weakest cell hits ~3.45V-3.50V. If you top balance you can set your charge controller to 3.45*16 and all cells will be charged to the same level but you will need a BMS to warn you or shut down the loads when the weakest cell gets close to 0%.

You are correct in saying that you only need a pack LVD if you bottom balance but this only holds true if the balance does not drift with time. Again this is unlikely and doesn't match my experience and experience of others. Of course the only way you can check if the battery has gone out of balance is to do another bottom balance.

Because of the very flat charge curve for an LFP battery, you cannot reliably charge an LFP battery to say 90% unless you have a constant current power supply. Solar is not a constant current supply.

Hi bill,I can't say I've heard of a single person bottom balancing their LFP cells, except EV guys. If you want to charge the bank to 55.2 volts (3.45v/cell) without a BMS then top balancing is the way to go. I agree with karrak that the reason for the increasing delta in voltage, as the cells are being charged, is slightly differing cell capacities. I have read that they can vary as much a 5%. So we use the top 80% or the bottom 80%. I believe it is easier to manage charging with top balancing because as the bank nears the end of absorb, all the cells are nearing the same voltage. As they are discharged, they will stay fairly close, probably within 10 millivolts, down to 20% SOC. Could be even lower, however I can't say for sure because I haven't taken mine below 20%. Something to consider is, deeper discharge means shorter life, so I try to keep usage between 70 and 80% of capacity. Charge to 99+% then down to 20 to 30%.It's easy to get freaked out when you see the cells coming so close to overcharging. Top balance does away with that concern. Over a year of daily cycling and my cells voltage delta is 25 millivolts at the end of absorb.

If you bottom-balance, you will be slightly ragged at the top. That is to be expected. When the first cell of your bank reaches your target voltage, your HVD is supposed to shut off charge. Some of the simpler setups will note when the first cell hits the upper limit, and take a bank-voltage reading, and use that as the HVD cutoff.

But you must choose either top or bottom - you can't combine both techniques. Bottom is great for high-current EV's and other high-draw applications. But here, as a solar storage application that typicall draws no more than 0.1C or less (to get through many day's autonomy), top balance is the most practical choice, as our relatively low current draw gives us PLENTY of time for a bank-level LVD to react, if it is set conservatively.

I have a nominal battery bank of 51.2 v, shall I set the inverter cutoff at 51.2 v to avoid over discharge ? The charger cutoff at 56.8? Is this healthy?

This battery will balance itself if you charge it to 56.8V, I assume you have four of the 12V batteries in series? It may extend the lifespan of the battery to only charge to 55.2V and charge to 56.8V and hold at this voltage for an hour or so say once a month to trigger the inbuilt cell balancers. You may be able to use the equalise charge setting on your charge controller to do this. What model charge controller are you using?

Another question, it gets quite hot in summer. Around 45 c. Would I need to cool the batteries? Would it be a good idea to seperate the charger and inverter from the batteries to reduce excess heat?

The graph below from this paper http://jes.ecsdl.org/content/145/10/3647.full.pdf+html gives an idea about how temperature effects the lifespan of LFP batteries.We too have hot summers where the temperature gets to 45C. The amount of time that the temperature is above 45C is fairly low and the average daily temperature is in the mid 20s. Under these conditions I don't think the losses will be too severe.

I agree with @nickdearing88 that you should take whatever steps you can to keep the battery temperature as low as possible but above 0C.

The graph below from this paper http://jes.ecsdl.org/content/145/10/3647.full.pdf+html gives an idea about how temperature effects the lifespan of LFP batteries.We too have hot summers where the temperature gets to 45C. The amount of time that the temperature is above 45C is fairly low and the average daily temperature is in the mid 20s. Under these conditions I don't think the losses will be too severe.

Simon;This appears to be a very useful chart - are there similar, easily understood charts of the popular EV LI battery types? NMC, LMO, NCA, etc.?Couple of questions;1. The "x" axis - Can it be interpreted as => a 1 ahr cell will degrade approx 5% with 4000 total ahrs (both discharge and recharge through it) at 15 oC? (And at 60oC, may not even make 4000 total ahrs!)?2. The chart likely does not take into effect idle time at SOC/temperatures (i.e., a LFP battery sitting for two years at a certain temperature and SOC may lose "throughput" also)?

Just trying to understand better the basic degradation mechanisms in my Nissan Leaf (LMO/NMC) battery (understanding that the "dynamic" use would be a so interdependent of power cycling, cell operating temperatures, ambient temperatures, idle time at certain SOC, etc, etc, make actual, real life predictions very difficult).

Couple of questions;1. The "x" axis - Can it be interpreted as => a 1 ahr cell will degrade approx 5% with 4000 total ahrs (both discharge and recharge through it) at 15 oC? (And at 60oC, may not even make 4000 total ahrs!)?

Yes, that is how I read it.

2. The chart likely does not take into effect idle time at SOC/temperatures (i.e., a LFP battery sitting for two years at a certain temperature and SOC may lose "throughput" also)?

I think this is one of the big factors affecting battery life. Leaving any lithium ion batteries idle at high SOC at high temperature will diminish their lifespan. As you can see from the graphs its is particularly true for NMC batteries.

Just trying to understand better the basic degradation mechanisms in my Nissan Leaf (LMO/NMC) battery (understanding that the "dynamic" use would be a so interdependent of power cycling, cell operating temperatures, ambient temperatures, idle time at certain SOC, etc, etc, make actual, real life predictions very difficult).

As you say it is difficult to make real life predictions. From my understanding, cycling a lithium ion battery between ~70%-30%, storing at low SOC, keeping the temperature down and limiting the charge and discharge current will lead to the best life span. One has to balance lifespan against utilisation. Comply with the manufacturers recommendations and you will get in excess of ten years use and/or the rated number of charge/discharge cycles out of the battery.

Have you seen anything on LFP or NMC that discusses how accurate a BMS Soc reading is near the end of warranty?I know that the battery stays very efficient near the end of life as it loses its capacity but does that affect Soc accuracy?

The Leaf is a car by Nissan. I have charged one here and it was a fun ride up into our park near here.

Have you seen anything on LFP or NMC that discusses how accurate a BMS Soc reading is near the end of warranty?I know that the battery stays very efficient near the end of life as it loses its capacity but does that affect Soc accuracy?

The Leaf is a car by Nissan. I have charged one here and it was a fun ride up into our park near here.

Hi Dave,Finally some rain and the end of the fire season. Have had some big fires in our corner of the state in the last week! All the best for your fire season!!!

I haven't seen anything specific about BMS SOC accuracy over the life of a battery. It will depend on how the coulomb/current efficiency changes as the battery ages and whether the BMS takes this change into account. My BMS software dynamically adjusts for any efficiency changes that might occur, hopefully the commercial units will do likewise. I have noticed an increase in efficiency of ~0.1% in my battery over the past three years, although this might just be inaccuracies in obtaining the data. This matches information I have read that implies that the efficiency will improve as the SEI layer thickens and stops unwanted side reactions. It ties in with this rather nice graph from here which shows the rate of decrease in capacity of the Tesla batteries decreasing with time/use.

Thanks for the info! Very nice to hear that as I was told that a couple years ago by an engineer at LG. I was translating Korean on google and there were some pauses that I thought weird. He said the same thing with the efficiency and I was skeptical after 25 years of lead acid and how it just degrades right from the start.

Nice to be past the fire season hump! Where I live the fire danger from wildfire is 2 weeks after the last rain. It almost never rains in the next 4 months so we are knee deep in it now. In the old days the insurance guys would look on google earth to actually see the terrain and how safe you had made the dwelling. Now they just say no.

It seems the Insurance companies learned nothing last year from the really bad fires. They said they were going to do physical inspections of the homes and base their rates on how safe the exterior of the home was. I think they just thought it was cheaper to raise everyone's rates.